class_docstr: NDArray trace object
  
  Parameters
  ----------
  name: str
      Name of backend. This has no meaning for the NDArray backend.
  model: Model
      If None, the model is taken from the `with` context.
  vars: list of variables
      Sampling values will be stored for these variables. If None,
      `model.unobserved_RVs` is used.
class_docstr_tok: ["NDArray","trace","object","Parameters","--","--","--","--","--","name",":","str","Name","of","backend",".","This","has","no","meaning","for","the","NDArray","backend",".","model",":","Model","If","None",",","the","model","is","taken","from","the","`","with","`","context",".","vars",":","list","of","variables","Sampling","values","will","be","stored","for","these","variables",".","If","None",",","`","model.unobserved_RVs","`","is","used","."]
code: def __len__(self):
      if not self.samples:  # `setup` has not been called.
          return 0
      return self.draw_idx
  
file_path: pymc/pymc/backends/ndarray.py
imports: from typing import Any, Dict, List, Optional
  import numpy
  from pymc.backends import base
  from pymc.backends.base import MultiTrace
  from pymc.model import Model, modelcontext
label_desc: Use image node_id 6 for calling the NDArray obj's underlying member method code with example usage: obj.__len__() and returns: self, int